ENH: remove prior scope level theme before applying theme at the same scope level

[vt5491]

Let's say for instance you're applying a "file type" scope theme against all '.html' files. Let's say you pick a dark theme, but then change your mind and want to apply a green theme e.g. 'atom-spring-green-syntax'. If you apply another file-type theme the prior dark theme (at the file-type level) is not removed first. Thus you end up with two file-type themes applied which can cause cross-interence (the resultant theme might be a hybrid of both due to the 'cascading' property of cascading style sheets).

What is usually wanted in this scenario is to replace the prior version with the new version, not overlay it. This is also applicable to file scope as well, but it's more noticeable at the 'file-type' scope since the interference can affect several editor instances.

This will be particularly problematic if you're trying out several themes, e.g. you don't have some previously worked out scheme where you apply a favorite theme, that you know looks good, at the file scope level.

[vt5491]

This is something that I, as the author of the package, never noticed since I rarely ever re-apply themes at the file-type scope level. I mostly apply at the file scope level, and it's possible I do remove the prior theme at this level, since I often re-apply themes at the file scope level and have never noticed any cross-contamination. However, I just had a scenario where I had to re-apply at file-type-scope and I had interference, so I obviously do not do this scope level.

The only workaround is go to atom commands (ctrl-shift-p), type "mta" and select "Multi Theme Applicator: Reset". This has the downside of removing all themes, but it will at least allow you to re-apply a single 'file-type' theme without any interference from any prior applications.

But this I think is a good suggestion, and is worthy of a dot release as soon as I can get around to it.

[vt5491]

Of course, I just realized, another workaround is to simply use the 'remove scoped theme' button before re-applying the new theme.